Electronic compasses have been installed in consumer electronic products to improve performance. For example, electronic compasses are applied to Global Positioning Systems (GPS) to improve positioning ability. In GPS, the forward direction is determined by measuring the movement of an object. However, the GPS cannot accurately position the object when the object moving at a low speed, or stationary. The electronic compasses can provide azimuth information for determining the forward direction.
Many schemes for sensing magnetic fields have been proposed, such as typical Hall devices or magneto-resistive devices. The magneto-resistive devices include anisotropic magneto-resistors (ARMs), giant magneto-resistors (GMRs) and tunneling magneto-resistors (TMRs). Compared with the Hall devices, the magneto-resistive devices have good sensitivity, and the back-end fabrication process for the magneto-resistive devices is easily integrated with the front-end CMOS fabrication process.
A typical TMR for a magnetic field sensor 95 is shown in FIGS. 1A to 1B. The TMR 95 includes a bottom plate of conducting metal, serving as a bottom electrode 102 formed on a substrate 90, an MTJ (Magnetic Tunneling Junction) device 110 formed on the bottom electrode 102 and a top plate of conducting material, serving as a top electrode 106 formed on the MTJ device 110. From the structural pattern of the MTJ device, one can define a cross having an intersection at the center, wherein the longer length is called a major axis 101 and the shorter length is called a minor axis 103. A line called an easy-axis 180 is collinear with a major axis 101. The MTJ device 110 includes a pinned layer 112, a tunneling layer 115 and a free layer 116, in which the MTJ device 110 is sandwiched between the bottom electrode 102 and the top electrode 106, for example. The pinned layer 112 is made of magnetic material formed on the bottom electrode 102 and has a first pinned magnetization 114, parallel to a pinned direction. The tunneling layer 115 of non-magnetic material is formed on the pinned layer 112. The free layer 116 of magnetic material is formed on the tunneling layer 115 and has a first free magnetization 118, initially parallel to the easy axis 180.
After the MTJ device is formed (i.e. after magnetic thin film stacking and pattern etching), the pinned direction is set by applying a magnetic field thereto during an annealing process. After the annealing process, the pinned direction will be parallel to the direction of the applied magnetic field, and the free magnetization tends to be parallel to the easy-axis due to the shape anisotropy. Therefore, the magnetic field sensing direction of the TMR is perpendicular to the easy-axis 180.
Through AMR or even GMR, an integrated horizontal 2-axis magnetic field sensor can be achieved, but their footprint sizes are quite large. Because of a very low resistivity, the device length has to be long enough to provide a usable value for sensing magnetic fields. FIGS. 2A and 2B are drawings, schematically illustrating a Wheatstone bridge circuit without and with shielding. As shown in FIG. 2A, the Wheatstone bridge circuit is a popular method for transforming the sensed magnetic field into an electronic signal. For the AMR magnetic sensor, each element R11, R21, R12, R22 of the bridge circuit is a series connection of several Barber pole biased AMRs and the shorting bar angles of adjacent elements are complementary, so that the bridge circuit is symmetric and full range operable. However, for the GMR or TMR magnetic field sensor, due to their symmetric magneto-resistance and magnetic field characteristics, two elements R21, R12 therefore must be shielded, as shown in FIG. 2B, and the bridge circuit only performs half range operation. For TMRs having high magneto-resistance ratio, the asymmetric half range operation of the bridge circuit results in losing linearity and accuracy of the output signal.